How Long Does it Take Sunlight to Reach the Earth?
Written by Fraser Cain
Sunlight takes just over 8 minutes to reach the Earth.
On average, the Earth is 150 million kilometers (93 million miles) away from the Sun. Its orbit is actually slightly elliptical, so can get a little closer and a little more distant from the Sun. That sounds like a long way, and it is.
But light travels really really fast. The speed of light in a vacuum is 300,000 km/s. How long does it take for sunlight to reach Earth? It only takes about 8 minutes.
In other words, if there's a flare on the surface of the Sun, the light will propagate out from the Sun. This light will sweep past Mercury in 3.22 minutes, Venus in 6.01 minutes, and reach Earth 8.32 minutes later. Neptune, the most distant planet, takes 4.16 hours for light to reach. The nearest star, Proxima Centauri, takes light 4.3 years to get to.
The fastest spacecraft ever launched from Earth is NASA's New Horizons mission, which will make a close encounter with Pluto and its moons in 2015. New Horizons is traveling at 75,000 km/h (47,000 mph), and would take 82 days to get from the Earth to the Sun.
One interesting way to think about the speed of light is to imagine what would happen if the Sun mysteriously, and instantaneously disappeared. Since light takes 8.32 minutes to get from the Sun to the Earth, we wouldn't know that the Sun was gone for just over 8 minutes after it had actually happened.
Filed under: Astronomy
